Communication system, base station and mobile station used in the communication system, and base station switching method

ABSTRACT

There are provided an OFDMA communication system capable of reducing a time necessary for a handover without releasing a wireless communication with a base station as a handover source to search base stations, a base station and a mobile station used in the communication system, and a base station switching method. 
     In an OFDMA communication system in which data communication is performed between a plurality of base stations  10  and a plurality of mobile stations, each base station  10  includes subchannel assignment setting unit  15 - 1  which sets assignment of subchannels including a first subchannel and a second subchannel for each of the mobile stations, the first subchannel including information indicating usable or unusable subchannels of the respective mobile stations and being assigned to the respective mobile stations, the second subchannel including actually-used data. When a request for switching the first subchannel is received from one of the mobile stations  20 , the second subchannel is collected and assigned to a same slot as the first subchannel assigned to the one of the mobile stations  20.

CROSS-REFERENCE TO THE RELATED APPLICATIONS

This application is a national stage of international application No.PCT/JP2007/070856 filed on Oct. 25, 2007, which also claims the benefitof priority under 35 USC §119 to Japanese Patent Application No.2006-293240 filed on Oct. 27, 2006 and Japanese Patent Application No.2006-293242filed on Oct. 27, 2006, the entire contents of all of whichare incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to an OFDMA communication system, a basestation and a mobile station used in the communication system, and abase station switching method for handover in the communication system.

BACKGROUND ART

As a radio access scheme of a PHS system or the like, a TDMA (TimeDivision Multiple Access)/TDD (Time Division Duplex) scheme has beenemployed in which a TDMA system and a TDD system are combined. Recently,an OFDMA (Orthogonal Frequency Division Multiplexing Access) systememploying an OFDMA scheme based on an OFDM (Orthogonal FrequencyDivision Multiplexing) technique has been proposed.

The OFDM is a scheme of dividing a carrier for data modulation into aplurality of “subcarriers” (subdivided carriers) orthogonal to eachother and distributing and transmitting a data signal in eachsubcarrier.

Hereinafter, an overview of the OFDM scheme will be described.

FIG. 15 is a block diagram illustrating a configuration of the OFDMmodulation device used at a transmission side. Transmission data isinput to the OFDM modulation device. The transmission data is suppliedto a serial/parallel conversion unit 201 and converted into dataincluding a plurality of low-speed transmission symbols. That is, aplurality of low-speed digital signals are generated by dividingtransmission information. This parallel data is supplied to an inversefast Fourier transformation (IFFT) unit 202.

The parallel data is allocated to each subcarrier configuring OFDM andmapped in a frequency domain. Here, the subcarrier is modulated by BPSK,QPSK, 16QAM, 64QAM, or the like. The mapping data is converted fromfrequency-domain transmission data to time-domain transmission data byperforming an IFFT operation. Accordingly, multi-carrier modulationsignals into which a plurality of subcarriers orthogonal to each otherare modulated independently are generated. An output of the IFFT unit202 is supplied to a guard interval addition unit 203.

As shown in FIG. 16, the guard interval addition unit 203 sets a rearpart of an effective symbol of transmission data as a guard interval andadds its copy to a front part of an effective symbol period for eachtransmission symbol. A base-band signal obtained by the guard intervaladdition unit is supplied to an orthogonal modulation unit 204.

The orthogonal modulation unit 204 orthogonally modulates a base-bandOFDM signal supplied from the guard interval addition unit 203 using acarrier signal supplied from a local oscillator 205 of the OFDMmodulation device, and performs frequency conversion into intermediatefrequency (IF) signal or a radio frequency (RF) signal. That is, afterfrequency-converting the base-band signal into a desired transmissionfrequency band, the orthogonal modulation unit outputs it to atransmission path.

FIG. 17 is a block diagram illustrating a configuration of an OFDMdemodulation device to be used in a receiving side. An OFDM signalgenerated by the OFDM modulation device of FIG. 15 is input to the OFDMdemodulation device through a predetermined transmission path.

An OFDM reception signal input to the OFDM demodulation device issupplied to the orthogonal demodulation unit 211. The orthogonaldemodulation unit 211 orthogonally demodulates the OFDM reception signalusing a carrier signal supplied from a local oscillator 212 of the OFDMdemodulation device, performs frequency conversion from a RF signal oran IF signal into a base-band signal, and obtain a base-band OFDMsignal. The OFDM signal is supplied to a guard interval removing unit213.

The guard interval removing unit 213 removes a signal added by the guardinterval addition unit 203 of the OFDM modulation device according to atiming signal supplied from a symbol timing synchronization unit (notshown). A signal obtained by the guard interval removing unit 203 issupplied to a fast Fourier transformation (FFT) unit 214.

The FFT unit 214 performs conversion to frequency-domain reception databy performing an FFT operation on input time-domain reception data.De-mapping is performed in the frequency domain and parallel data isgenerated for each subcarrier. Here, the demodulation to the modulationof BPSK, QPSK, 16QAM, 64QAM, or the like performed for each subcarrieris performed. Parallel data obtained by the FFT unit 214 is supplied toa parallel/serial conversion unit 215 and output as reception data.

The OFDM is a scheme for dividing a carrier into a plurality ofsubcarriers. The OFDMA is a scheme for collecting and grouping aplurality of subcarriers among the subcarriers in the OFDM andperforming multiplex communication by allocating one or more groups toeach user. Each group is called a subchannel. That is, each userperforms a communication using one or more allocated subchannels.According to a communication data amount, a propagation environment, andthe like, subchannels are adaptively increased and decreased, andallocated.

In a PHS system employing such OFDMA scheme, a frame is configured so asto include, for example, four time slots, where the vertical axisrepresents a frequency and the horizontal axis represents a time. Adownlink period and an uplink period are both divided into a pluralityof frequency bands in the frequency axis. The subchannel allocated tothe first frequency band is called control subchannel and is used as acontrol channel (CCH). The other frequency bands are configured astraffic channels (TCH) each including a plurality of subchannels.

When a mobile station moves to a boundary of a cell which is a rangewirelessly communicable with a base station or when radio waves from thebase station in communication are weakened to disable the communication,switching to a base station having strong radio waves in another cell isperformed after or before the radio waves are weakened, which is calleda handover.

Such a handover is performed as follows, for example, as described inPatent Document 1.

FIG. 18 shows a situation in which a wireless communicable range with abase station BTS 101 is a cell 112 and a wireless communicable rangewith a base station BTS 102 is a cell 113, a mobile station MS 107 ismoving from the cell 112 of the base station BTS 101 as a handoversource to the cell 113 of the base station BTS 102 as a handoverdestination.

The mobile station MS 107 monitors the power of the mobile station MS107 during communication with the base station BTS 101 and notifies acenter MSC 111 of the monitoring result when a handover to the basestation BTS 102 is predicted. The mobile station MS 107 receives a listof neighbor base stations, monitors signals from the neighbor basestations at a constant time interval on the basis of the list, andnotifies the base station BTS 101 of the monitoring result. When aboundary condition of the handover is satisfied, a message istransmitted to a base station controller BSC 109. The message includes aparameter for recognizing the mobile station MS and data on a newchannel (time slot) to be used in a communication between the mobilestation MS and the base station BTS 102 from now on. The handover to thebase station BTS 102 is started under the control of the MSC 111 whenthe mobile station MS 107 is in the overlap range of the cell 112 andthe cell 113, and the handover is ended when the mobile station 107enters the cell 113.

Patent Document 1: JP-A-2002-300628 (pages 3 and 4 and FIG. 10)

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

In the communication system employing the above-mentioned OFDMA scheme,since a communication can be made without using the entire bandwidth ofthe communication system, a mobile station only using a narrowcommunication bandwidth which is a part of the communication system canbe used. The mobile station using a narrow communication bandwidth canreduce a manufacturing cost thereof, compared with a mobile stationusing the entire bandwidth of the communication system.

In the communication system employing the OFDMA scheme, a controlchannel (CCH) uses a specific slot. Accordingly, when a communication isperformed through a subchannel apart in frequency from the controlchannel, the mobile station using a narrow communication bandwidthcannot receive information through the control channel during thecommunication and thus cannot control a handover using the controlchannel.

There is a great difference in reception level between the controlchannel and the traffic channel. Accordingly, when a mobile stationusing a wide communication bandwidth and receiving information throughboth the traffic channel and the control channel receives information inthe same slot and amplifies signals in accordance with a reception levelof one of the channels in communication, information may not receivedthrough the other channel. That is, the control channel in the same slotas the traffic channel during a communication may not be utilized.

Therefore, when a handover is necessary, base stations as a handoverdestination have to be searched using a control channel after releasingthe wireless connection with the base station as a handover source.However, in this case, the time necessary for the handover is elongated.

Accordingly, instantaneous interruption may occur in the communicationand call cutoff may occur at the worst.

The present invention is made to solve the above-described problems, andan object of the present invention is to provide an OFDMA communicationsystem capable of reducing a time necessary for a handover withoutreleasing a wireless communication with a base station as a handoversource and searching base stations when a mobile station using a narrowcommunication bandwidth make a communication through a subchannel apartin frequency from a control channel or searches a base station as ahandover destination using the control channel in the same slot as atraffic channel in communication, a base station and a mobile stationused in the communication system, and a base station switching method.

Means for Solving the Problem

In order to solve the problem, according to the present invention, thereis provided an OFDMA communication system in which data communication isperformed between a plurality of base stations and a plurality of mobilestations, wherein each of the base stations comprises a subchannelassignment setting unit which sets assignment of subchannels including afirst subchannel and a second subchannel for each of the mobilestations, the first subchannel including information indicating usableor unusable subchannels of the respective mobile stations and beingassigned to the respective mobile stations, the second subchannelincluding actually-used data, and wherein when a request for switchingthe first subchannel is received from one of the plurality of mobilestations, the second subchannel is collected and assigned to a same slotas the first subchannel assigned to the one of the mobile station (claim1).

According to a system of the present invention, there is provided anOFDMA communication system in which data communication is performedbetween a plurality of base stations and a plurality of mobile stations,wherein each of the base stations comprises a subchannel assignmentsetting unit which sets assignment of subchannels including a firstsubchannel and a second subchannel for each of the mobile stations, thefirst subchannel including information indicating usable or unusablesubchannels of the respective mobile stations and being assigned to therespective mobile stations, the second subchannel includingactually-used data, and wherein when a request for switching the firstsubchannel is received from one of the plurality of mobile stations, atleast one slot is released for a control channel used by a base station(claim 2).

According to the present invention, there is provided an OFDMAcommunication system in which data communication is performed between aplurality of base stations and a mobile station, the communicationsystem comprising: a mobile station state determining unit whichdetermines a state of the mobile station; a handover-destination basestation searching unit which searches base stations as handoverdestination candidates when the mobile station state determining unitdetermines that the mobile station is in a perch state; a communicationstate measuring unit which measures a communication state with the basestations as the handover destination candidates; and a handoverdestination list preparing unit which prepares a handover destinationlist including a priority order of the base stations as the handoverdestination candidates on the basis of the communication state measuredby the communication state measuring unit, wherein when a handoverbecomes necessary, a connection between the mobile station and the basestations as the handover destination candidates is tried in accordancewith the priority order in the handover destination list (claim 3).

According to the present invention, a time stamp of date and time whenthe list is prepared is recorded in the handover destination list, andwhen a plurality of the handover destination lists exist, a handoverdestination list having a newest time stamp is used (claim 4).

According to the present invention, there is provided a base stationcomprising: a subchannel assignment setting unit which sets assignmentof subchannels including a first subchannel and a second subchannel foreach of a plurality of mobile stations, the first subchannel includinginformation indicating usable or unusable subchannels of the respectivemobile stations and being assigned to the respective mobile stations,the second subchannel including actually-used data, wherein when arequest for switching the first subchannel is received from one of theplurality of mobile stations, information on subchannel assignment inwhich the second subchannel is collected and assigned to a same slot asthe first subchannel is transmitted to the one of the mobile stations(claim 5).

According to the present invention, there is provided a base stationused in the communication system of claim 3 or 4, wherein when acommunication data amount decreases to no transmission data, the basestation connects with a mobile station using only a first subchannelincluding information indicating usable or unusable subchannels of themobile station and which is assigned to the mobile station (claim 6).

According to the present invention, there is provide a mobile stationused in the communication system of claim 1 or 2, the mobile stationcomprising a communication state measuring unit which measures acommunication state with a base station as a switching destination and aswitching-destination base station searching unit which searches a basestation as a switching destination (claim 7).

According to the present invention, there is provided a mobile stationin an OFDMA communication system, the mobile station comprising: amobile station state determining unit which determines a state of themobile station; a handover-destination base station searching unit whichsearches base stations as handover destination candidates when themobile station state determining unit determines that the mobile stationis in a perch state; a communication state measuring unit which measuresa communication state with the base stations as the handover destinationcandidates; a handover destination list preparing unit which prepares ahandover destination list including a priority order of the basestations as the handover destination candidates on the basis of thecommunication state measured by the communication state measuring unit;and a handover destination list storage unit which stores the handoverdestination list while adding a time stamp of date and time when thebase stations as the handover destination candidates are searched (claim8).

According to the present invention, there is provided a base stationswitching method for a handover in the communication system of claim 1or 2, the method comprising: transmitting a request for switching thefirst subchannel from a mobile station to a base station as a switchingsource when a downlink radio quality degrades; collecting and assigningthe second subchannel to a same slot as the first subchannel in the basestation as the switching source, and transmitting information onsubchannel assignment in which the first subchannel and the secondsubchannel are collected in the same slot, to the mobile station;receiving the information on the subchannel assignment and searchingneighbor base stations using all relative slots not used incommunication by the mobile station; and setting up a link between themobile station and a base station as a switching destination detected bythe searching and cutting off a link between the mobile station and thebase station as the switching source (claim 9).

According to the present invention, there is provided a base stationswitching method for a handover in the communication system of claim 1or 2, the method comprising: transmitting a handover instruction from abase station as a switching source to a mobile station when an uplinkradio quality degrades; receiving the handover instruction andtransmitting a request for switching the first subchannel from themobile station to the base station as the switching source; collectingand assigning the second subchannel to a same slot as the firstsubchannel in the base station as the switching source, and transmittinginformation on subchannel assignment in which the first subchannel andthe second subchannel are collected in the same slot, to the mobilestation; receiving the information on the subchannel assignment andsearching neighbor base stations using all relative slots not used incommunication by the mobile station; and setting up a link between themobile station and a base station as a switching destination detected bythe searching and cutting off a link between the mobile station and thebase station as the switching source (claim 10).

According to the present invention, there is provided a base stationswitching method for a handover in the communication system of claim 3or 4, the method comprising: selecting a base station as a switchingdestination using the handover destination list by the mobile stationwhen a downlink radio quality degrades; and setting up a link betweenthe mobile station and the selected base station as the switchingdestination and cutting off a link between the mobile station and a basestation as a switching source (claim 11).

According to the present invention, there is provided a base stationswitching method for a handover in the communication system of claim 3or 4, the method comprising: transmitting a handover instruction from abase station as a switching source to a mobile station when an uplinkradio quality degrades; receiving the handover instruction and selectinga base station as a switching destination using the handover destinationlist in the mobile station; and setting up a link between the mobilestation and the selected base station as the switching destination andcutting off a link between the mobile station and the base station asthe switching source (claim 12).

Advantage of the Invention

According to the invention, when a first subchannel switching request isreceived from a mobile station, second subchannels are collected andassigned to a same slot as the first subchannel assigned to the mobilestation, whereby it is possible to search the base station as a handoverdestination using a slot other than the slot in which the firstsubchannel and the second subchannels are collected.

A mobile station in communication can transmit and receive informationthrough a control channel by releasing at least one slot for use in abase station.

According to the invention, in the communication system employing anOFDMA scheme, by searching candidate base stations as a handoverdestination and previously preparing a handover destination list inwhich priority as a handover destination is given to the candidate basestations when a mobile station is in a perch state, it is possible toaccess the optimal base station included in the list with reference tothe handover destination list without searching the base stations when ahandover is necessary.

Therefore, when a mobile station using a narrow communication bandwidthmake a communication through a subchannel apart in frequency from acontrol channel or searches base stations as a handover destinationusing the control channel in the same slot as a traffic channel incommunication and a handover is necessary, it is possible to reduce atime required for the handover without releasing a wirelesscommunication with the base station as a handover source and searchingthe base stations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a configuration of an OFDMA frame usedin a communication method according to a first embodiment of theinvention.

FIG. 2 is a diagram schematically illustrating a handover method betweena mobile station and plural base stations in a communication systemaccording to the first embodiment of the invention.

FIG. 3 is a block diagram illustrating a configuration of a base stationin the communication system according to the first embodiment of theinvention.

FIG. 4 is a block diagram illustrating a configuration of a mobilestation in the communication system according to the first embodiment ofthe invention.

FIG. 5 is a diagram illustrating a handover sequence started by themobile station in the communication system according to the firstembodiment of the invention.

FIG. 6 is a diagram illustrating a handover sequence started by the basestation in the communication system according to the first embodiment ofthe invention.

FIG. 7 is a block diagram illustrating a configuration of a base stationin a communication system according to a second embodiment of theinvention.

FIG. 8 is a block diagram illustrating a configuration of a mobilestation in a communication system according to a second embodiment ofthe invention.

FIG. 9 is a diagram illustrating an example of a handover destinationlist.

FIG. 10 is a block diagram illustrating a flow of a handover destinationlist preparing process.

FIG. 11 is a diagram illustrating a sequence between a mobile stationand a base station from the start of preparing the handover destinationlist to the end thereof.

FIG. 12 is a flow diagram illustrating a handover sequence in thecommunication system according to the second embodiment of theinvention.

FIG. 13 is a diagram illustrating the handover sequence started by themobile station in the communication system according to the secondembodiment of the invention.

FIG. 14 is a diagram illustrating the handover sequence started by thebase station in the communication system according to the secondembodiment of the invention.

FIG. 15 is a block diagram illustrating a configuration of an OFDMmodulator used in a transmission party.

FIG. 16 is a diagram illustrating a guard interval.

FIG. 17 is a block diagram illustrating a configuration of an OFDMmodulator used in a reception party.

FIG. 18 is a diagram illustrating a configuration of a past mobilestation network and a state where a mobile station moves from a cell ofa base station to a cell of another base station.

DESCRIPTION OF REFERENCE NUMERALS AND SIGNS

10, 10A, 10B: base station

10-1, 10-2, 10-3, 10-4, 10-5, 10-n: base station

11: wireless communication unit

12: signal processing unit

13: modulation and demodulation unit

14: external I/F unit

15: control unit

15-1: subchannel assignment setting unit (subchannel assignment settingmeans)

20, 20A, 20B: mobile station

21: wireless communication unit

22: signal processing unit

23: modulation and demodulation unit

24: input and output unit

25: control unit

25-1: communication state measuring unit (communication state measuringmeans)

25-2: handover-destination base station searching unit(handover-destination base station searching means)

25-3: mobile station measuring unit (mobile station state measuringmeans)

25-4: handover destination list preparing unit (handover destinationlist preparing means)

25-5: handover destination list storage unit

BEST MODE FOR CARRYING OUT THE INVENTION

Before describing embodiments of the present invention, a frameconfiguration of an OFDMA scheme used in a communication system and acommunication method according to embodiments of the invention will befirst described with reference to FIG. 1.

In the frame configuration shown in FIG. 1, for example, four time slots(S1 to S4) are used in a PHS system, the vertical axis representsfrequency, and the horizontal axis represents time. In FIG. 1, adownlink period (communication from a base station to a mobile station)and an uplink period (communication from a mobile station to a basestation) are both divided into 28 frequency bands with respect to thefrequency axis. The subchannel of the first frequency band is called acontrol subchannel and is used as a control channel (CCH). The controlsubchannel instructs which subchannel of the respective time slotsshould be used in each frequency band.

FIG. 1 shows an example of the PHS system, where four base stations canbe designated by the control subchannels C₁ to C₄. In the PHS system,information is intermittently transmitted through the control channelsevery 100 ms.

The remaining 27 frequency bands include traffic subchannels T₁ to T₁₀₈for transmitting and receiving data. 108 subchannels in total areprovided by the number of 27 in the frequency axis direction and thenumber of 4 in the time axis direction.

The traffic subchannels include an anchor subchannel as a firstsubchannel and an extra subchannel as a second subchannel.

The anchor subchannel (first subchannel) is a subchannel which is usedto notify the respective mobile stations of subchannels assigned to themobile stations, and which is used to allow the base station and themobile station to negotiate whether data is correctly exchanged in are-transmission control.

The extra subchannel (second subchannel) is a subchannel fortransmitting actually-used data and plural extra subchannels can beassigned to one mobile station. In this case, as the number of assignedextra subchannels increases, the bandwidth becomes wider, therebyenabling a faster communication.

Subchannel assignment information indicating which subchannels should beassigned to communication between each mobile station and a base stationis called MAP information, which is notified from the base station tothe mobile stations in advance.

Hereinafter, a communication system according to embodiments of thepresent invention will be described in detail with reference to theaccompanying drawings. FIG. 2 is a diagram schematically illustrating ahandover method between a mobile station and plural base stations in acommunication system according to a first embodiment and a secondembodiment of the present invention.

In FIG. 2, a mobile station 20 is in communication with a base station10-1 while using an anchor subchannel of slot S2. When the mobilestation 20 moves to a boundary of a range where the mobile station canwirelessly communicate with the base station 10-1 or when radio wavesfrom the base station 10-1 in communication are weakened to disable thecommunication for other reasons, a handover for switch to a base stationhaving stronger radio waves need to be performed after or before theradio waves are weakened.

In the case of FIG. 2, it is assumed that plural neighbor base stations10-2 to 10-5 exist. When the mobile station 20 is in a perch state, themobile station can communicate through only the anchor subchannel andthus can receive a control channel (CCH) in three time slots other thanthe slot used in the anchor subchannel. For example. in FIG. 2, the basestations 10-2, 10-3, and 10-5 is enabled in handover (HO) and the basestation 10-4 is disabled in handover (HO).

First Embodiment

Configurations of a mobile station and a base station used in acommunication system and a communication method according to the firstembodiment of the present invention will be described.

FIG. 3 is a block diagram illustrating a configuration of a base stationand FIG. 4 is a block diagram illustrating a configuration of a mobilestation.

As shown in FIG. 3, a base station 10A (base station 10 according to thefirst embodiment) includes a wireless communication unit 11 which isconnected to an antenna, converts a signal from a signal processing unit12 to be described later into an RF signal and converts the received RFsignal into a signal which can be treated by the signal processing unit12, a signal processing unit 12 which processes a received signal or atransmission signal, a modulation and demodulation unit 13 whichmodulates and demodulates a signal, an external I/F unit 14 which isconnected to an upper-level communication network, and a control unit 15which controls the signal processing unit 12 and the modulation anddemodulation unit 13.

The control unit 15 includes a subchannel assignment setting unit 15-1which sets assignment of subchannels including an anchor subchannel(ASCH) and extra subchannels (ESCH) for each of the mobile stations. Theanchor subchannel includes information indicating usable or unusablesubchannels of each mobile station and being assigned to each mobilestation and the extra subchannels (ESCH) includes actually-used data.

The subchannel assignment setting unit 15-1 collects and assigns theextra subchannels (ESCH) to the same slot as the anchor subchannel(ASCH) when receiving an anchor subchannel (ASCH) switching request froma mobile station.

Accordingly, the base station 10A transmits to the mobile stationsubchannel assignment information (MAP information) indicating that theanchor subchannel (ASCH) and the extra subchannels (ESCH) are collectedin one slot.

As shown in FIG. 4, a mobile station 20A (mobile station 20 according tothe first embodiment) includes a wireless communication unit 21 which isconnected to an antenna and converts a signal from a signal processingunit 22 to be described later into an RE signal or converts the receivedRF signal into a signal which can be treated by the signal processingunit 22, a signal processing unit 22 which processes a received signalor a transmission signal, a modulation and demodulation unit 23 whichmodulates or demodulates a signal, an input and output unit 24 forinputting and outputting data or sound, and a control unit 25 whichcontrols the signal processing unit 22 and the wireless communicationunit 21.

The control unit 25 includes a communication state measuring unit 25-1which measures a communication state (for example, SINR, RSSI, and thelike) with a base station as a switching destination for a handover, anda handover-destination base station searching unit 25-2 which searches abase station as a switching destination.

A handover sequence in the communication system according to the firstembodiment of the invention will be described. First, the handoversequence started by a mobile station in the communication systemaccording to this embodiment will be described with reference to FIG. 5.

-   -   When downlink radio quality degrades, for example, when the SINR        (Signal-to-Interference and Noise Ratio) is equal to or less        than a threshold value as a reference, the handover sequence        starts.

The determination on the degradation of the radio quality may depend onan RSSI (Received Signal Strength Indicator) or the like using about 10dBμV as a threshold value.

-   -   The mobile station 20A transmits an anchor subchannel (ASCH)        switching request to the base station 10-1 as a switching        source.    -   The base station 10-1 as a switching source collects and assigns        the extra subchannels (ESCH) to the same slot as the anchor        subchannel (ASCH) under the control of the subchannel assignment        setting unit 15-1. In this way, subchannel assignment        information (MAP information) indicating that the anchor        subchannel (ASCH) and the extra subchannels (ESCH) are included        in one slot is transmitted to the mobile station.    -   The mobile station 20A searches neighbor base stations through        all the relative slots (slots to which the anchor subchannel        (ASCH) and the extra subchannels (ESCH) are not assigned at all)        not used for communication.    -   The mobile station 20A transmits a link channel (LCH) setup        request to a base stations 10-n (a base station other than the        base station 10-1) as the switching destination detected by the        search.    -   The base station 10-n as the switching destination selects an        anchor subchannel (ASCH) and transmits the assignment of the        link channel (LCH) to the mobile station, when receiving the        link channel (LCH) setup request.    -   The mobile station 20A and the base station 10-n as the        switching destination start a communication through the anchor        subchannel (ASCH) of the base station 10-n as the switching        destination. In the communication of the base station 10-n as        the switching destination through the anchor subchannel (ASCH),        any of the downlink communication and the uplink communication        may be first performed.    -   When receiving the MAP information for assigning the extra        subchannels (ESCH) through the anchor subchannel (ASCH) of the        base station 10-n as the switching destination, the mobile        station 20A considers that the handover is successful and cuts        off the link to the base station 10-1 as the switching source.

The handover sequence started by a base station in the communicationsystem according to the first embodiment will be described withreference to FIG. 6.

-   -   When uplink radio quality degrades, for example, when the SINR        (Signal-to-Interference and Noise Ratio) is equal to or less        than a threshold value as a reference, the handover sequence is        started.

The determination on the degradation of the radio quality may depend onan RSSI (Received Signal Strength Indicator) or the like using about 10dBμV as a threshold value.

-   -   The base station 10-1 as a switching source transmits a handover        instruction to the mobile station 20A.    -   The mobile station having received the instruction transmits an        anchor subchannel (ASCH) switching request to the base station        as a switching source.    -   The base station 10-1 as a switching source collects and assigns        the extra subchannels (ESCH) to the same slot as the anchor        subchannel (ASCH) under the control of the subchannel assignment        setting unit 15-1. In this way, subchannel assignment        information (MAP information) indicating that the anchor        subchannel (ASCII) and the extra subchannels (ESCH) are included        in one slot is transmitted to the mobile station.    -   The mobile station 20A searches neighbor base stations through        all the relative slots (slots to which the anchor subchannel        (ASCII) and the extra subchannels (ESCH) are not assigned at        all) not used for communication.    -   The mobile station 20A transmits a link channel (LCH) setup        request to a base stations 10-n (a base station other than the        base station 10-1) as the switching destination detected by the        search.    -   The base station 10-n as the switching destination selects an        anchor subchannel (ASCII) and transmits the assignment of the        link channel (LCH) to the mobile station, when receiving the        link channel (LCH) setup request.    -   The mobile station 20A and the base station 10-n as the        switching destination start a communication through the anchor        subchannel (ASCH) of the base station 10-n as the switching        destination. In the communication of the base station 10-n as        the switching destination through the anchor subchannel (ASCH),        any of the downlink communication and the uplink communication        may be first performed.    -   When receiving the MAP information for assigning the extra        subchannels (ESCH) through the anchor subchannel (ASCH) of the        base station 10-n as the switching destination, the mobile        station 20A considers that the handover is successful and cuts        off the link to the base station 10-1 as the switching source.

In the handover sequence started by the mobile station or the handoversequence started by the base station, when the base station receives theanchor subchannel switching request from the mobile station, at leastone slot may be released for a control channel used by a base station.

When it is determined by the mobile station that the radio quality isnot degraded to be equal to or less than the threshold value at the timeof receiving the MAP information indicating that the anchor subchannel(ASCII) and the extra subchannels (ESCH) are included in one slot, themobile station may perform only the process of searching the basestations as the handover destinations.

Second Embodiment

Configurations of a mobile station and a base station used in acommunication system and a communication method according to the secondembodiment of the invention will be described.

FIG. 7 is a block diagram illustrating a configuration of a base stationand FIG. 8 is a block diagram illustrating a configuration of a mobilestation.

As shown in FIG. 7, a base station 10B (base station 10 according to thesecond embodiment) includes a wireless communication unit 11 which isconnected to an antenna and converts a signal from a signal processingunit 12 to be described later into an RF signal or converts the receivedRF signal into a signal which can be treated by the signal processingunit 12, a signal processing unit 12 which processes a received signalor a transmission signal, a modulation and demodulation unit 13 whichmodulates or demodulates a signal, an external I/F unit 14 which isconnected to an upper-level communication network, and a control unit 15which controls the signal processing unit 12 and the modulation anddemodulation unit 13.

The control unit 15 includes a subchannel assignment setting unit 15-1setting the assignment of subchannels used in communication to themobile station.

The subchannel assignment setting unit 15-1 assigns the extrasubchannels depending on the communication data amount when data istransmitted and received therethrough (in an active state), but does notassign the extra subchannels when the communication data amountdecreases to no transmission data. In this case, the base stationconnects with the mobile station using only the anchor subchannel. Thestate where the mobile station is connected with the mobile stationusing only the anchor subchannel is called as a perch state.

As shown in FIG. 8, a mobile station 20B (mobile station 20 according tothe second embodiment) includes a wireless communication unit 21 whichis connected to an antenna and converts a signal from a signalprocessing unit 22 to be described later into an RF signal or convertsthe received RF signal into a signal which can be treated by the signalprocessing unit 22, a signal processing unit 22 which processes areceived signal or a transmission signal, a modulation and demodulationunit 23 which modulates or demodulates a signal, an input and outputunit 24 for inputting and outputting data or sound, and a control unit25 which controls the signal processing unit 22 and the wirelesscommunication unit 21.

The control unit 25 includes a communication state measuring unit 25-1which measures a communication state (for example, SINR, RSSI, and thelike) with a base station as a handover destination, ahandover-destination base station searching unit 25-2 which searchesbase stations as a handover destination, a mobile station statedetermining unit 25-3 which determines whether the mobile station 20B isin the active state or the perch state, a handover destination listpreparing unit 25-4 which prepares a list of base stations as handoverdestinations, and a handover destination list storage unit 25-5 whichstores the handover destination list including a list of base stationsto which priority is given in the order suitable as a handoverdestination and which are arranged in the priority order and time stampsof date and time of the search on the basis of the search result of thehandover-destination base station searching unit 25-2.

FIG. 9 shows an example of the handover destination list stored in thehandover destination list storage unit 25-5.

Information elements of the handover destination list include the timestamps (date and time) when the base stations as the handoverdestination are searched and identifiers (CS-ID) of the base stations.

The priority of the handover destination is given to the searched basestations on the basis of the communication state measured by thecommunication state measuring unit 25-1 and the searched base stationsare stored.

In preparing the handover destination list shown in FIG. 9, the priorityof the handover is given from the base station having the better SINR onthe basis of the measured communication state with the base stations,for example, the measured SINR (Signal-to-Interference and Noise Ratio),and the SINR values are not included in the handover destination listfor the purpose of reducing the memory amount. An old handoverdestination list in which a predetermined time passes after thepreparation thereof is removed or determined as being invalid withreference to the time stamp.

As the communication state with the base station, an RSSI (ReceivedSignal Strength Indicator) or the like may be measured.

The flow of a handover destination list preparing process will bedescribed with reference to FIG. 10.

The mobile station state determining unit 25-3 detects that the state ofthe mobile station 20B is changed from the active state as a normalcommunication state where data is transmitted and received to a statewhere the data amount decreases and transmission data does not exist,that is, the perch state where the extra subchannels are not assignedand the mobile station is connected using only the anchor subchannel(step S1). When the mobile station is changed to the perch state (YES instep S1), it is determined whether the handover destination list existsin the handover destination list storage unit 25-5 (step S2).

When the handover destination list exists (YES in step S2), the timestamp of the handover destination list is compared with the current timeto determine whether a predetermined time passes after previouslypreparing the list (step S3).

When the handover destination list does not exist (NO in step S2) orwhen a predetermined time passes after previously preparing the list(YES in step S3), the process of step S4 is performed. On the otherhand, when a predetermined time does not pass after previously preparingthe list (NO in step S3), the process of step S5 to be described lateris performed.

The handover-destination base station searching unit 25-2 searches theneighbor base stations using the control channels (CCH) of the slotsother than the slot to which the anchor subchannel is assigned (stepS4).

When the search ends, the handover destination list preparing unit 25-4prepares the handover destination list (step S5). The prepared handoverdestination list is stored in the handover destination list storage unit25-5.

The handover destination list is prepared as follows.

The communication state measuring unit 25-1 measures the communicationstates (for example, SINR, RSSI, and the like) of the base stationssearched in step S4. The base station of which the measuredcommunication state is equal to or more than a predetermined thresholdvalue is called a handover-enable base station, and the handover-enablebase stations are ordered from the base station having a bettercommunication state. The handover-disable base stations should not beincluded in the list. The time stamps of the search time in step S5 andthe identifiers (CS-ID) of the base stations are described to obtain ahandover destination list.

The mobile station state determining unit 25-3 determines whether themobile station is kept in the perch state (step S6).

When the perch state is maintained (YES in step SG), the process of stepS2 is performed again. On the other hand, when the perch state ends (NO)in step S6), the communication state is changed to the active state(step S7).

A sequence performed between the mobile station and the base stationfrom the start of the preparation of the handover destination list tothe end thereof will be described with reference to FIG. 11.

-   -   When the communication state between the mobile station and the        base station is active and the MAP information included in the        anchor subchannel and transmitted from the base station does not        include the assignment of extra subchannels, the communication        state is changed to the perch state.    -   After the communication state is changed to the perch state, the        mobile station starts the flow of searching the base stations as        the handover destination, as shown in FIG. 10. When the search        of the base stations as the handover destination in all the        other slots in this period ends, the search process is once        stopped at that time. When this period is continued after the        search process ends and a predetermined time passes after the        previous search time, the flow of searching the base stations as        the handover destination is performed again.    -   When the MAP information included in the anchor subchannel and        transmitted from the base station includes the assignment of the        extra subchannels, the flow of searching the base stations as        the handover destination is stopped. At this time, the handover        destination list is prepared as shown in FIG. 10.    -   The communication state is changed to the active state.

The handover sequence in the communication system according to thisembodiment will be described.

First, a flow from the detection of the degradation in radio quality incommunication to the transmission of the link channel (LCH) setuprequest for the handover will be first described with reference to FIG.12.

As shown in FIG. 12, when the mobile station detects the degradation inradio quality (step S21), that is, when the SINR in the anchorsubchannel of a downlink is equal to or less than a reference value, themobile station determines whether a handover destination list exists(step S22). When the handover destination list exists (YES in step S22),a list having the newest time stamp (the time stamp closest to thecurrent time) is selected (step S23).

When only one handover destination list exists, the list is used. Then,it is determined whether base stations (CS-ID) are included in theselected handover destination list (step S24).

When base stations (CS-ID) are included in the list (YES in step S24),the link channel (LCH) setup request is transmitted to the base stationhaving the highest priority among the base stations included in theselected handover destination list (step S25).

It is determined whether a signal received from the base station inresponse to the link channel (LCH) setup request is a link channel (LCH)assignment-disable signal or a link channel (LCH) assignment-enablesignal (step S26).

When the link channel (LCH) assignment-enable signal is received, thecommunication with the base station is started (step S27).

On the other hand, when the link channel (LCH) assignment-disable signalis received, the base station having the highest priority is deletedfrom the handover destination list (CS-ID is deleted) (step S28). Then,the process of step S24 is performed again. That is, the base stationhaving the second highest priority transmits the link channel (LCH)setup request (when the assignment thereof is disabled, the basestations having the third highest priority, the fourth highest priority,. . . and the lowest priority in the list sequentially transmit the linkchannel (LCH) setup request).

The above-mentioned process may be ended in the base station havingpredetermined priority.

When the handover destination list does not exist (NO in step S22) orwhen the base stations (CS-ID) are not included in the selected handoverdestination list (NO in step S24), a handover request is transmitted tothe base station in communication (step S29). That is, it is notifiedthat a high-efficiency handover algorithm cannot be started.Accordingly, the handover algorithm is changed to current algorithm.

The handover sequence started by a mobile station in the communicationsystem according to this embodiment will be described with reference toFIG. 13.

-   -   When downlink radio quality degrades, for example, when the SINR        (Signal-to-Interference and Noise Ratio) is equal to or less        than a threshold value as a reference, the handover sequence        starts.

The determination on the degradation of the radio quality may depend onan RSSI (Received Signal Strength Indicator) or the like using about 10dBμV as a threshold value.

-   -   The mobile station 20B selects a base station 10-n (a base        station other then the base station 10-1) as the handover        destination (switching destination) by the flow shown in FIG. 10        using the above-mentioned handover destination list.    -   The link channel (LCH) setup request is transmitted to the        selected base station as the switching destination.    -   The base station 10-n as the switching destination selects the        anchor subchannel (ASCH) in response to the setup request and        transmits the link channel (LCH) assignment signal to the mobile        station.    -   The mobile station 20B and the base station 10-n as the        switching destination start a communication through the anchor        subchannel (ASCH) of the base station 10-n as the switching        destination. In the communication of the base station 10-n as        the switching destination through the anchor subchannel (ASCII),        any of the downlink communication and the uplink communication        may be first performed.    -   When receiving the MAP information for assigning the extra        subchannels (ESCH) through the anchor subchannel (ASCH) of the        base station 10-n as the switching destination, the mobile        station considers that the handover is successful and cuts off        the link to the base station 10-1 as the switching source.

The handover sequence started by a base station in the communicationsystem according to this embodiment will be described with reference toFIG. 14.

-   -   When uplink radio quality degrades, for example, when the SINR        (Signal-to-Interference and Noise Ratio) is equal to or less        than a threshold value as a reference, the handover sequence        starts.

The determination on the degradation of the radio quality may depend onan RSSI (Received Signal Strength Indicator) or the like using about 10dBμV as a threshold value.

-   -   The base station 10-1 as the switching source transmits a        handover instruction to the mobile station 20B. The mobile        station 20B having received this instruction selects a base        station 10-n (a base station other then the base station 10-1)        as the handover destination (switching destination) by the flow        shown in FIG. 8 using the above-mentioned handover destination        list.    -   The link channel (LCH) setup request is transmitted to the        selected base station 10-n as the switching destination.    -   The base station 10-n as the switching destination selects the        anchor subchannel (ASCH) in response to the setup request and        transmits the link channel (LCH) assignment signal to the mobile        station 20B.    -   The mobile station 20B and the base station 10-n as the        switching destination start a communication through the anchor        subchannel (ASCII) of the base station 10-n as the switching        destination. In the communication of the base station 10-n as        the switching destination through the anchor subchannel (ASCH),        any of the downlink communication and the uplink communication        may be first performed.    -   When receiving the MAP information for assigning the extra        subchannels (ESCH) through the anchor subchannel (ASCH) of the        base station 10-n as the switching destination, the mobile        station 20B considers that the handover is successful and cuts        off the link to the base station 10-1 as the switching source.

The present application is based on Japanese Patent Application No.2006-293240 filed on Oct. 27, 2006 and Japanese Patent Application No.2006-293242 filed on Oct. 27, 2006, the contents of which areincorporated herein by reference.

What is claimed is:
 1. An OFDMA communication system in which datacommunication is performed between a plurality of base stations and aplurality of mobile stations, wherein each of the base stationscomprises a subchannel assignment setting unit which sets assignment ofsubchannels including a first subchannel and a second subchannel foreach of the mobile stations, the first subchannnel including informationindicating usable or unusable subchannels of the respective mobilestations and being assigned to the respective mobile stations, thesecond subchannel including actually-used data, and wherein when arequest for switching the first subchannel is received from one of theplurality of mobile stations, the second subchannel is collected andassigned to a same slot as the first subchannel assigned to the one ofthe mobile station.
 2. A mobile station used in the OFDMA communicationsystem according to claim 1, the mobile station comprising: acommunication state measuring unit which measures a communication statewith a base station as a switching destination; and aswitching-destination base station searching unit which searches a basestation as a switching destination.
 3. A base station switching methodfor a handover in the OFDMA communication system according to claim 1,the method comprising: transmitting a request for switching the firstsubchannel from a mobile station to a base station as a switching sourcewhen a downlink radio quality degrades; collecting and assigning thesecond subchannel to a same slot as the first subchannel in the basestation as the switching source, and transmitting information onsubchannel assignment in with the first subchannel and the secondsubchannel are collected in the same slot, to the mobile station;receiving the information on the subchannel assignment and searchingneighbor base stations using all relative slots not used incommunication by the mobile station; and setting up a link between themobile station and a base station as a switching destination detected bythe searching and cutting off a link between the mobile station and thebase station as the switching source.
 4. A base station switching methodfor a handover in the OFDMA communication system according to claim 1,the method comprising: transmitting a handover instruction from a basestation as a switching source to a mobile station when an uplink radioquality degrades; receiving the handover instruction and transmitting arequest for switching the first subchannel from the mobile station tothe base station as the switching source; collecting and assigning thesecond subchannel to a same slot as the first subchannel in the basestation as the switching source, and transmitting information onsubchannel assignment in which the first subchannel and the secondsubchannel are collected in the same slot, to the mobile station;receiving the information on the subchannel assignment and searchingneighbor base stations using all relative slots not used incommunication by the mobile station; and setting up a link between themobile station and a base station as a switching destination detected bythe searching ad cutting off a link between the mobile station and thebase station as the switching source.
 5. An OFDMA communication systemin which data communication is performed between a plurality of basestations and a plurality of mobile stations, wherein each of the basestations comprises a subchannel assignment setting unit which setsassignment of subchannels including a first subchannel and a secondsubchannel for each of the mobile stations, the first subchannelincluding information indicating usable or unusable subchannels of therespective mobile stations and being assigned to the respective mobilestations, the second subchannel including actually-used data, andwherein when a request for switching the first subchannel is receivedfrom one of the plurality of mobile stations, at least one slot isreleased for a control channel used by a base station and the secondsubchannel is collected and assigned to a same slot as the firstsubchannel assigned to the one of the mobile station.
 6. An OFDMAcommunication system in which data communication is performed between aplurality of base stations and a mobile station, the communicationsystem comprising: a mobile station state determining unit whichdetermines a state of the mobile station a handover-destination basestation searching unit which searches base stations as handoverdestination candidates when the mobile station state determining unitdetermines that the mobile station is in a perch state; a communicationstate measuring unit which measures a communication state with the basestations as the handover destination candidates; and a handoverdestination list preparing unit which prepares a handover destinationlist including a priority order of the base stations as the handoverdestination candidates on the basis of the communication state measuredby the communication state measuring unit, wherein when a handoverbecomes necessary, a connection between the mobile station and the basestations as the handover destination candidates is tried in accordancewith the priority order in the handover destination list, wherein a basestation connects with a mobile station using a first subchannel and asecond subchannel, where in the first subchannel including informationindicating usable or unusable subchannels of the mobile station andwhich is assigned to the mobile station, the second subchannel includingactually used data; wherein when a request for switching the firstsubchannel is received from one of the plurality of mobile stations, thesecond subchannel is collected and assigned to a same slot as the firstsubchannel assigned to the one of the mobile station.
 7. The OFDMAcommunication system according to claim 6, wherein a time stamp of dateand time when the list is prepared is recorded in the handoverdestination list, and wherein when a plurality of the handoverdestination lists exist, a handover destination list having a newesttime stamp is used.
 8. A base station used in the OFDMA communicationsystem according to claim 6, wherein when a communication data amountdecreases to no transmission data, the base station connects with amobile station using only a first subchannel including informationindicating usable or unusable subchannels of the mobile station andwhich is assigned to the mobile station.
 9. A base station switchingmethod for a handover in the OFDMA communication system according toclaim 6, the method comprising: selecting a base station as a switchingdestination using the handover destination list by the mobile stationwhen a downlink radio quality degrades; and setting up a link betweenthe mobile station and the selected base station as the switchingdestination and cutting off a link between the mobile station and a basestation us a switching source.
 10. A base station switching method for ahandover in the OFDMA communication system according to claim 6, themethod comprising: transmitting a handover instruction from a basestation as a switching source to a mobile station when an uplink radioquality degrades; receiving the handover instruction and selecting abase station as a switching destination using the handover destinationlist in the mobile station; and setting up a link between the mobilestation and the selected base station as the switching destination andcutting off a link between the mobile station and the base station asthe switching source.
 11. A base station comprising; a subchannelassignment setting unit which sets assignment of subchannels including afirst subchannel and a second subchannel for each of a plurality ofmobile stations, the first subchannel including information indicatingusable or unusable subchannels of the respective mobile stations andbeing assigned to the respective mobile stations, the second subchannelincluding actually-used data, wherein when a request for switching thefirst subchannel is received from one of the plurality of mobilestations, information on subchannel assignment in which the secondsubchannel is collected and assigned to a same slot as the firstsubchannel is transmitted in the one of the mobile stations.
 12. Amobile station in an OFDMA communication system, the mobile stationcomprising: a mobile station state determining unit which determines astate of the mobile station; a handover-destination base stationsearching unit which searches base stations as handover destinationcandidates when the mobile station state determining unit determinesthat the mobile station is in a perch state; a communication statemeasuring unit which measures a communication state with the basestations as the handover destination candidates; a handover destinationlist preparing unit which prepares a handover destination list includinga priority order of the base stations as the handover destinationcandidates on the basis of the communication state measured by thecommunication state measuring unit; and a handover destination liststorage unit which stores the handover destination list while adding atime stamp of date and time when the base stations as the handoverdestination candidates are searched; wherein a base station connectswith a mobile station using a first subchannel and a second subchannel,where in the first subchannel including information indicating usable orunusable subchannels of the mobile station and which is assigned to themobile station, the second subchannel including actually used data;wherein when a request for switching the first subchannel is receivedfrom one of the plurality of mobile stations, the second subchannel iscollected and assigned to a same slot as the first subchannel assignedto the one of the mobile station.